Recently, more stringent regulations of particulate matter emitted by diesel engines have been passed in Europe and the United States. To meet these regulations, it is expected that particulate filters will be necessary.
These particulate filters need to meet multiple contradictory exacting requirements. For example, the filter must have sufficient porosity (generally greater than 55 percent porosity) while still retaining most of the emitted micrometer sized diesel particulates (generally greater than 90 percent capture of the emitted particulates). The filter must also be permeable enough so that back pressure is relatively low while the filter is in the clean state and also remains low while accumulating an amount of soot before being regenerated.
The filter must withstand the corrosive exhaust environment for long periods of time. The filter must have an initial strength to be compression fitted into a container attached to the exhaust system. The filter must be able to withstand thermal cycling (i.e., retain adequate strength) from the burning off of the soot entrapped in the filter (regeneration) over thousands of cycles where local temperatures may reach as high as 1600° C. and typically reach between 600 to 1000° C. under typical regeneration cycles.
The filter must, for long periods of time, withstand the corrosive exhaust environment containing water, nitrous oxide, carbon monoxide, carbon dioxide and hydrocarbons at elevated temperature. In addition, the filter must be stable to not only gaseous environment, but materials that are in contact with the filter such as catalyst and catalyst supports (washcoat particulates such as high surface area alumina) and ash from the exhaust such as alkali and alkaline earth oxides and other metal oxides, phosphates, and sulfates.
Porous ceramic filters generally are made from extrusions of ceramic particulates when heated bond together to form a porous ceramic body made up of many individual ceramic grains ceramically bonded together via a disordered/glassy grain boundary phase, ordered/crystalline grain boundary phase, or a combination thereof of differing composition than the individual ceramic grains. For example, cordierite in many instances has a glassy grain boundary phase. Mullite filters as well often have such glassy grain boundary phases such as those having interlaced crystals grown together have been used and are described by U.S. Pat. Nos. 5,098,455; 6,596,665; 7,528,087; and 7,425,297; and WO 92/11219. Silicon carbide, likewise often has a glassy grain boundary phase or silicon binding phase depending on the process used to sinter the grains together.
Accordingly, it would be desirable to provide both a formation method and a ceramic composition that meets or improves one or more of the aforementioned and in particular improves the thermal stability of such a composition.